1. Field of the Invention
The present invention relates to latex coating compositions for application to film substrate materials such as thermoplastic films, for example, oriented polypropylene films.
2. Background of the Related Art
Latex coating compositions are well-known for improving the surface qualities of thermoplastic films, such as oriented polypropylene and polyethylene films. A Latex is an aqueous dispersion of spherical polymer particles in the range of about 0.01 to about 1.0 micron (0.0004-0.04 mils) in diameter.
U.S. Pat. No. 4,058,649, the disclosure of which is incorporated by reference herein, describes the process of preparing a latex obtained by emulsion polymerization and its use for coating thermoplastic film substrates. The latex coating particles described in U.S. Pat. No. 4,058,649 contain approximately 80% vinylidene chloride, 10% methyl acrylate and 10% methacrylic acid forming two layered latex particles. The higher the concentration of vinylidene chloride, i.e. 80% or more, provides improved oxygen and vapor barrier qualities while the methyl acrylate improves heat sealability and surface qualities. However, at these concentrations the coated films exhibit high blocking and a high coefficient of friction which results in poor machinability and processability of the coated films in heat-sealing apparatus. In addition, the heat-seal strength of such films is low. Alternatively, if the vinylidene chloride concentration is lowered while the concentration acrylic esters is raised, the surface qualities, i.e., blocking, coefficient of friction and heat-seal strength increase. However, lowering of the vinylidene chloride concentration results in worsened barrier properties, i.e., transmission of gasses such as oxygen, vapors, and the like.
Another type of latex coating particle is disclosed in U.S. Pat. No 3,309,330. The latex coating particle has an inner terpolymer core layer having a relatively high vinylidene chloride concentration, i.e. 88-96% by weight with a relatively low alkyl acrylate and acrylic acid concentration, and an outer terpolymer coating portion having a relatively low vinylidene chloride concentration, i.e., from 60-88% by weight with a high concentration of acrylonitrile or alkyl acrylate or alkyl methyl acrylate, and acrylic or methacrylic acid. These latex particles provide a coating for thermoplastic films with good gas barrier properties as well as low surface friction and low blocking characteristics. Thermoplastic films coated with this type of latex coating possess good surface qualities and also have a relatively low minimum heat sealing temperature of approximately 103.degree. C. (or about 217.degree. F.) However, as shown in the table in column 5 of the specification of U.S. Pat. No 3,304,330, the coated film has a relatively low heat-seal strength, at about 140.degree. C. (or 284.degree. F.) of only 350 grams/inch. Although this heat-seal strength provides a vast improvement over a physical mixture of the components, such a low heat seal strength is not satisfactory for most packaging applications.
Accordingly, none of the related art provides a latex coating which imparts to a thermoplastic film improved oxygen and vapor barrier properties without sacrificing one or many of the desirable surface properties, such as heat seal strength, low minimum seal temperature, and/or machinability, e.g., low coefficient of friction and blocking characteristics.
It is therefore an object of this invention to provide a multi-layered copolymer particle latex coating which imparts to thermoplastic films coated therewith high barrier qualities in addition to providing superior surface qualities including blocking and friction characteristics without sacrificing desirable minimum sealing temperatures and high heat seal strength.